**Core Concept:** Leptin is a hormone secreted by adipose tissue, primarily from the white adipose tissue. It acts on the hypothalamus to regulate appetite, energy expenditure, and body weight. Leptin levels are inversely proportional to body fat mass, meaning that higher fat mass leads to lower leptin levels, and vice versa.

**Why the Correct Answer is Right:** Leptin is primarily secreted by adipocytes (fat cells) within white adipose tissue. As the fat mass increases, more leptin is produced and released into the bloodstream. This leptin then binds to specific receptors in the hypothalamus, which in turn decreases appetite and increases energy expenditure. This helps regulate body weight and overall energy homeostasis.

**Why Each Wrong Option is Incorrect:**
A. Leptin is not secreted by muscle tissue; this option is incorrect because muscle is a major energy reserve but does not produce significant amounts of leptin.
B. Leptin is not secreted by liver tissue; this option is incorrect as the liver plays a crucial role in glucose homeostasis but does not produce substantial leptin levels.
C. Leptin is not primarily secreted by the heart; this option is incorrect as the heart is mainly involved in circulation rather than producing hormones like leptin.
D. Leptin is not secreted by the gastrointestinal tract; this option is incorrect as the gastrointestinal tract is involved in digestion and absorption, not hormone production like leptin.

**Clinical Pearl:** Understanding the relationship between leptin levels and body weight is essential as leptin resistance has been linked to obesity and type 2 diabetes. Maintaining a healthy body weight helps ensure proper leptin signaling and overall physiological function.

**Correct Answer:** Leptin is primarily secreted by adipose tissue (fat cells).